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		<h1>Creating Core System Classes</h1>

		<p>Every time CodeIgniter runs there are several base classes that
			are initialized automatically as part of the core framework. It is
			possible, however, to swap any of the core system classes with your
			own versions or even extend the core versions.</p>

		<p>
			<strong>Most users will never have any need to do this, but
				the option to replace or extend them does exist for those who would
				like to significantly alter the CodeIgniter core.</strong>
		</p>

		<p class="important">
			<strong>Note:</strong>&nbsp; Messing with a core system class has a
			lot of implications, so make sure you know what you are doing before
			attempting it.
		</p>


		<h2>System Class List</h2>

		<p>The following is a list of the core system files that are
			invoked every time CodeIgniter runs:</p>

		<ul>
			<li>Benchmark</li>
			<li>Config</li>
			<li>Controller</li>
			<li>Exceptions</li>
			<li>Hooks</li>
			<li>Input</li>
			<li>Language</li>
			<li>Loader</li>
			<li>Log</li>
			<li>Output</li>
			<li>Router</li>
			<li>URI</li>
			<li>Utf8</li>
		</ul>

		<h2>Replacing Core Classes</h2>

		<p>
			To use one of your own system classes instead of a default one simply
			place your version inside your local
			<dfn>application/core</dfn>
			directory:
		</p>

		<code>
			application/core/
			<dfn>some-class.php</dfn>
		</code>

		<p>If this directory does not exist you can create it.</p>

		<p>Any file named identically to one from the list above will be
			used instead of the one normally used.</p>

		<p>
			Please note that your class must use
			<kbd>CI</kbd>
			as a prefix. For example, if your file is named
			<kbd>Input.php</kbd>
			the class will be named:
		</p>

		<code>
			class CI_Input {<br /> <br /> }
		</code>



		<h2>Extending Core Class</h2>

		<p>If all you need to do is add some functionality to an existing
			library - perhaps add a function or two - then it's overkill to
			replace the entire library with your version. In this case it's
			better to simply extend the class. Extending a class is nearly
			identical to replacing a class with a couple exceptions:</p>

		<ul>
			<li>The class declaration must extend the parent class.</li>
			<li>Your new class name and filename must be prefixed with <kbd>MY_</kbd>
				(this item is configurable. See below.).
			</li>
		</ul>

		<p>
			For example, to extend the native
			<kbd>Input</kbd>
			class you'll create a file named
			<dfn>application/core/</dfn>
			<kbd>MY_Input.php</kbd>
			, and declare your class with:
		</p>

		<code>
			class MY_Input extends CI_Input {<br /> <br /> }
		</code>

		<p>Note: If you need to use a constructor in your class make sure
			you extend the parent constructor:</p>

		<code>
			class MY_Input extends CI_Input {<br /> <br />
			&nbsp;&nbsp;&nbsp;&nbsp;function __construct()<br />
			&nbsp;&nbsp;&nbsp;&nbsp;{<br />
			&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;parent::__construct();<br />
			&nbsp;&nbsp;&nbsp;&nbsp;}<br /> }
		</code>

		<p class="important">
			<strong>Tip:</strong>&nbsp; Any functions in your class that are
			named identically to the functions in the parent class will be used
			instead of the native ones (this is known as "method overriding").
			This allows you to substantially alter the CodeIgniter core.
		</p>

		<p>If you are extending the Controller core class, then be sure to
			extend your new class in your application controller's constructors.</p>

		<code>
			class Welcome extends MY_Controller {<br /> <br />
			&nbsp;&nbsp;&nbsp;&nbsp;function __construct()<br />
			&nbsp;&nbsp;&nbsp;&nbsp;{<br />
			&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;parent::__construct();<br />
			&nbsp;&nbsp;&nbsp;&nbsp;}<br /> <br />
			&nbsp;&nbsp;&nbsp;&nbsp;function index()<br />
			&nbsp;&nbsp;&nbsp;&nbsp;{<br />
			&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;$this->load->view('welcome_message');<br />
			&nbsp;&nbsp;&nbsp;&nbsp;}<br /> }
		</code>

		<h3>Setting Your Own Prefix</h3>

		<p>
			To set your own sub-class prefix, open your
			<dfn>application/config/config.php</dfn>
			file and look for this item:
		</p>

		<code>$config['subclass_prefix'] = 'MY_';</code>

		<p>
			Please note that all native CodeIgniter libraries are prefixed with
			<kbd>CI_</kbd>
			so DO NOT use that as your prefix.
		</p>




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